HSV-1 infection induces strain-specific transcriptional differences in infected neurons

Why do some people with cold sores around their lips have painful lesions, while others have no symptoms, while still spreading the virus? These differences could be due to variations in how certain strains of herpes simplex (HSV-1) – the virus that causes cold sores, as well as genital herpes – activate, a new study at Penn State finds. gene expression in neurons. .

HSV-1 affects more than half of the world’s population. Not only does it cause recurring problems, such as cold sores and genital herpes, but recent research has implicated chronic infection with HSV-1 with the development of disease later in life, including neurodegenerative diseases such as Alzheimer’s disease. “

Moriah Szpara, Associate Professor of Biology, Biochemistry and Molecular Biology, Penn State

Szpara explained that the life cycle of HSV-1 begins when it comes into contact with mucous surfaces, where it invades skin cells, replicates and can induce the formation of local lesions. The virus also enters local nerve endings in the skin and passes through neurons in the nervous system. There the virus can remain dormant until it reactivates on future occasions. Neuronal damage and host immune responses triggered by viral reactivations are thought to contribute to long-term neurodegeneration.

“Since each person carries a subtly different version of HSV-1, this could explain some of the variations in human responses to infection; for example, why people have different triggers for their outbreaks or why some people experience more painful wounds. Differences in the frequency of viral outbreaks or virus-induced gene expression patterns could also affect the different rates at which people with chronic infections develop neurodegenerative diseases. “

To study the causes of this variation in responses, Szpara and his colleagues infected human neuronal cells with one of three strains of HSV-1 that are known to differ in their ability to cause disease in the nervous system. Then, they used deep sequencing to identify and quantify the transcriptomes – the complete set of messenger RNAs (mRNAs) made in a cell at any given time – of neurons when infected with HSV-1.

According to Szpara, when a neuronal cell becomes infected with HSV-1, the resulting transcriptome includes the entire collection of mRNAs produced by both the human neuron and the HSV-1 virus. By examining when and how much mRNA is expressed during infection, scientists can gain information about the proteins that will soon be produced from these mRNAs. It is the viral proteins and new viral progeny produced during infection that ultimately lead to health problems.

“By simultaneously examining viral and neuronal transcriptomes in infected cells, we were able to observe the interaction between the timing of viral mRNA production and protein production, and the resulting host responses,” said Szpara said.

Scientists used two additional techniques – immunofluorescence staining of neurons and Western blot for viral protein levels – to observe the results of viral and host gene expression. They also used scanning electron microscopy to directly observe changes in neuronal morphology during infection.

Their findings appeared online in PLOS pathogens March 22.

Researchers have found that different genetic variants of HSV-1 induce different patterns of gene expression in human neuronal cells. Specifically, they found that the viral variants expressed their genes at different rates and amounts, which likely contributes to the different timing and severity of symptoms in hosts. For example, they found that one of the variants, which shows lower virulence in animal models, displays a different and apparently delayed rate of expression of viral and host genes in neurons. In another example, they found that a variant caused larger changes in the expression of genes involved in cell adhesion (or the ability of cells to attach to each other), which could impact on cell-to-cell propagation of HSV-1.

The team also found that these different patterns of gene expression depended on whether the infected cell was a neuron or some type of skin cell.

“Together, these data demonstrate the importance of studying factors specific to the viral strain and cell type that may contribute to neurovirulence in vivo. It also highlights the specificity of HSV-1-host interactions, ”Szpara said. “Our study suggests that the differences observed between viral variants in cellular models like neurons can be used to help understand the more complex interactions of viruses with hosts.”